ORA-29273: HTTP request failed intermittent error using the utl_http package - sql

Question

I'm using the utl_http package to make HTTP GET requests to an IIS site on the same server (local) as Oracle. Sometimes it works and I get the response, but more often than not it hangs for about 15 seconds and then I get this error:
ORA-29273: HTTP request failed ORA-06512: at "SYS.UTL_HTTP", line 1722 ORA-29263: HTTP protocol error
As a test, I've got a small static text file in the IIS site, so this is how I'm testing it:
select utl_http.request('http://domain.com/test.txt') from dual
I get the same problem if I run it in Oracle Apex instead of direct on the db.
The other thing I've tried is to create a package of my own that does the HTTP request using the long utl_htp.begin_request() method, instead of the utl_http.request() shortcut. This gives the exact same problem (works sometimes but errors mostly - same error).
The pattern I'm seeing is if I wait a while and then try, it works for the first 2-10 times, and then begins erroring. When it does work, I get the response instantly and when it errors, there is always the delay before the error.
If I request the text file URL (or any other resource in the site) using a remote web browser then I get the correct response every time.
I have tried setting a timeout like below but it doesn't have any effect. For example instead of timing out after 3 seconds it continues for 10 or 15 seconds before the error is shown.
UTL_HTTP.set_transfer_timeout(3);
I think I can rule out ACL because it works sometimes.
Does anyone know what might cause this behaviour?

Answer 1

Possible reasons
-> You may have a problem with your TNS-Listener.
From the command prompt window, try to run TNSPING service_name .. try to run it quickly several times and check if it fails in some of them.
I had once a similar problem. Try to re-configure your TNS-Listener.
There must be also an option in which you can give an IP number in the TNS-listener definition. This also solves sometimes these kind of problems.
-> IIS problem.

Answer 2

Read about SET_PERSISTENT_CONN_SUPPORT Procedure:
https://docs.oracle.com/cd/B28359_01/appdev.111/b28419/u_http.htm#i1027673
Using: utl_http.set_persistent_conn_support(true, 30);

Answer 3

Could you be exceeding the limit of of concurrent HTTP connections? I vaguely remembering that I run into a similar problem when I forgot to close the HTTP connection.